Optical data storage systems provide a means for storing great quantities of data on a disk. The data is accessed by focusing a laser beam onto the data layer of the disk and then detecting the reflected light beam. Various kinds of systems are known. In a ROM (Read Only Memory) system, such as a compact disk system (CD-ROM), data is permanently embedded as marks in the disk at the time of manufacture of the disk. The data is detected as a change in reflectivity as the laser beam passes over the data marks. A WORM (Write-Once Read-Many) system allows the user to write data by making marks, such as pits, on a blank optical disk surface. Once the data is recorded onto the disk it cannot be erased. The data in a WORM system is also detected as a change in reflectivity. There are also erasable optical data storage systems, such as phase change and magneto-optic (M-O) systems. While phase change systems also read data by sensing a change in reflectivity, M-O systems read data by measuring the rotation of the incident polarization caused by the M-O) media.
To increase the storage capacity of an optical disk, multiple data layer systems have been proposed. An optical disk having two or more data layers may be accessed at different layers by changing the focal position of the lens. One example of this type of system is described in U.S. Pat. No. 5,202,875 assigned to IBM. In such systems, the light from the laser source must pass through one or more of the light-transmissive disk substrates in order to access the data layers on the different substrates. The focusing of the light through a relatively thick substrate introduces spherical aberration into the light beam. In the case of conventional single-disk optical systems, this can be corrected by slightly changing the curvature of the focusing lens by a fixed amount because the amount of substrate material through which the light must pass remains fixed. However, in multiple data layer optical disk systems, because the light is required to be focused through different numbers and thicknesses of substrates, depending on which data layer is being accessed, some form of adjustable active compensation for spherical aberration is required.